1. Field of the Invention
The present invention relates to a lens barrel for a video camera, and a linear feeding system used therefor. More particularly, the invention relates to a lens barrel for a video camera, and a linear feeding system used for a zoom lens feeding mechanism or the like for a lens barrel of a video camera, for example.
2. Related Background Art
Conventionally, a linear feeding system of the kind has used a DC motor with brush or a stepping motor as a source of its driving force, for example.
Among the linear feeding systems described above, there has been the system wherein an optical encoder is coupled outside its driving source in order to control the rotation of the driving source in a high precision.
However, with respect to this type of linear feeding system, the following problem has been encountered:
For a DC motor with brush adopted as described above as a driving source, a system should be prepared to reduce its rotational speed by use of gears while the DC motor is kept revolving at a high speed.
In this system, backlash is caused by the gears thus used so that the feeding accuracy is lowered inevitably. At the same time, noises deriving from the gears are comparatively great, among other problems.
Also, when a DC motor is used as a driving source for a system wherein the motor is controlled by means of an optical encoder, another problem is encountered that the encoder should be installed externally, making it necessary to arrange its coupling section highly accurately, leading to the increased costs of parts and assembling.
Further, there is a problem that the torque of a small DC motor is not good enough to allow an external encoder to be coupled directly for driving the motor.
Meanwhile, if a stepping motor is used as a source of driving force, there is also encountered a problem, among others, that the accuracy of stop positions is degraded because its driving control is an open loop. Moreover, the adoption of a stepping motor makes it difficult to sense the loss of synchronism, resulting in a defective performance in some cases. Particularly when micro steps are performed by use of a stepping motor, the position of a part to be driven cannot be known during the entire period of one step, hence making the execution of exact positioning difficult.